GPs are being asked to reduce or axe activities including childhood immunisation campaigns, out-of-hours care and minor surgery in emerging evidence that £20bn of NHS "efficiency savings" are leading to cuts in patient care.

Evidence of the cuts has appeared in the form of a letter to GPs in south London, in which health managers say they have to make £2.32m savings in 2011-13, and list services that should be reduced or abolished this financial year.

Further services, including physiotherapy and osteopathy, are being reviewed in anticipation of possible further cuts in future years.

Azeem Majeed, one of the GPs whose practice received the letter, calculates that the cut of just over £100,000 for this year amounts to about 10% of the surgery's core budget – a figure he estimates will be about average for GPs in the area affected.

"The concern is those practices will offer poorer services," said Majeed. "The government would say these are local decisions, they wash their hands of it. PCTs [primary care trusts, which until April decide much of GPs' budgets] would say we have got to have changes, we have got to cut money, because we have got to make £20bn of savings from somewhere."

Although there has been a growing clamour of complaints about services suffering from funding cuts, closures and drug rationing, among other problems, health managers have usually argued that these changes should not affect the overall quality of patient care because treatments are being delivered more efficiently or through another part of the NHS. The cuts to these GP services, however, appear to be clear reductions in the availability of services, and in some cases the end of them.

Majeed, also professor of primary care at Imperial College, London, warns that many of the cuts could create higher costs elsewhere: for example, if fewer children are immunised against once common and dangerous childhood diseases like measles, mumps and rubella or whooping cough – which has seen a surge in the number of cases in the past year – or because patients will have to have testing or minor surgery at hospitals, where costs tend to be higher.

Other affected services this year include translation – for literature such as health promotion leaflets, or in person – which has been replaced by a telephone helpline, and "near patient testing" in GP surgeries for problems such as high cholesterol, glucose and HIV.

Such concerns are magnified by what Majeed's practice estimates will be a further 5-20% of cuts over the next two or three years.

"People will start to go to A&E: that costs a lot more and puts a strain on hospitals," he said. "And there's evidence from a number of studies that the better access to GPs is, the better you get in terms of low rates of inpatient admissions for things like strokes [and] lung infections. You end up with poorer health, more cases of hospital care and higher costs."

The letter from the primary care trust for Lambeth was sent to 47 GP practices in the area, and relates to personal medical services, which the authority authorises and pays GPs to deliver.

It comes as there have been increasing claims from around the country about the impact of the "Nicholson challenge", the warning by the NHS chief executive, David Nicholson, in 2009 that by the middle of this decade the health service would need to have cut £20bn from the cost of its services because of the failing economy, increasing numbers of patients and the rising cost of drugs and new treatments.

A Guardian investigation last year uncovered reports that services were suffering because birth centres were closing; patients were being denied pain-relieving drugs; leaflets advising parents on how to care for newborn babies and avoid cot death were rationed or scrapped; people were receiving fewer visits from health visitors and district nurses; and support for problem drinkers was restricted.

The GPs' trade journal, Pulse, has reported that practices in Staffordshire had called for a review earlier this year after the PCT said it needed to cut its budget for personal medical services by £800,000 – leading to warnings of redundancies among GPs. . And elsewhere in London individual GP surgeries reported being asked to reduce their budgets.

Earlier this month, the general practitioners' committee of the British Medical Association also warned that a proposed new GP contract could lead to the biggest funding cuts in the profession's history, though negotiations with government are still at an early stage.

A Department of Health spokesperson said: "The NHS budget is being increased in real terms and while efficiencies must be made to meet future challenges, there is no excuse for simply cutting services patients need.

"Many trusts are saving money and improving services. We would expect south-east London to do the same."

A spokesman for the south-east London PCT cluster, into which the Lambeth PCT has been merged in preparation for being replaced by clinical commissioning groups (CCGs) in April, said the letter was part of a review of priorities, which has included residents and health professionals.

"In 2009-10 we identified a number of areas of our commissioned services, including in primary care, to test if we are getting the best possible outcomes for local patients from our existing investment. Our initial assessment identified up to £2.32m that could be saved from GP local enhanced services contracts without impacting on patient quality or outcomes to be achieved over a period of two years.

"This included underspending budgets where the takeup by practices had been historically low. Following further analysis and review we have now reduced this assessment to £1.5m, of which £1m has been implemented to date, without impacting detrimentally on patient outcomes or on service quality."

The authority said it had spent £900,000 to improve primary and community care services offered through GPs.

The spokesman added: "Our review has given us the opportunity to refocus and better incentivise the provision of primary care contracts to be more closely aligned towards to our priority health goals, which seek to address the highest health needs in Lambeth, as prioritised by local people and health professionals. These include: stopping smoking, improving blood pressure management, promoting good mental health, earlier detection and management of HIV, improving care for people with diabetes, reducing childhood obesity and preventing harm from alcohol."